This disclosure relates to flow batteries for selectively storing and discharging electric energy.
Flow batteries, also known as redox flow batteries or redox flow cells, are designed to convert electrical energy into chemical energy that can be stored and later released when there is demand. As an example, a flow battery may be used with a renewable energy system, such as a wind-powered system, to store energy that exceeds consumer demand and later release that energy when there is greater demand.
A basic flow battery includes a redox flow cell having a negative electrode and a positive electrode separated by an ion-exchange membrane or a non-conductive separator filled with electrolyte. A negative electrolyte is delivered to the negative electrode and a positive electrolyte is delivered to the positive electrode to drive an electrochemically reversible redox reaction. Upon charging, the electrical energy supplied causes a chemical reduction reaction in one electrolyte and an oxidation reaction in the other electrolyte. The ion-exchange membrane prevents the electrolytes from mixing rapidly but permits selected ions to pass through to complete the redox reactions while electrically isolating the two electrodes. Upon discharge, the chemical energy contained in the electrolyte is released in the reverse reactions and electrical energy can be drawn from the electrodes. Flow batteries are distinguished from other electrochemical devices by, inter alia, the use of externally-supplied, liquid electrolytes on at least on side that participate in a reversible electrochemical reaction.